Your last attempt is definitely not correct, but your previous code is working just fine:
Doing so you get a new dataset fdecteamstats with the column WL appended to it, like so:
library(dplyr)
decteamstats <- tibble::tribble(
~ID, ~Home, ~Team, ~Mins, ~oMins, ~dMins, ~oPOSS, ~dPOSS, ~ORTG, ~DRTG, ~NETRTG, ~PTS, ~dPTS, ~FGA, ~dFGA,
"UCLA", 0L, "Seattle U", 39L, 20L, 19L, 72L, 72L, 72L, 108L, -36L, 52L, 78L, 59L, 51L,
"Long Beach St.", 0L, "Seattle U", 38L, 22L, 17L, 76L, 77L, 99L, 104L, -5L, 75L, 80L, 64L, 59L,
"Washington", 0L, "Seattle U", 39L, 21L, 18L, 68L, 69L, 60L, 106L, -46L, 41L, 73L, 59L, 56L,
"Northwest (WA)", 1L, "Seattle U", 39L, 16L, 23L, 70L, 69L, 127L, 58L, 69L, 89L, 40L, 59L, 58L,
"Col. of Idaho", 1L, "Seattle U", 39L, 19L, 20L, 68L, 70L, 115L, 77L, 38L, 78L, 54L, 63L, 53L,
"California", 0L, "Seattle U", 39L, 20L, 19L, 63L, 64L, 103L, 109L, -6L, 65L, 70L, 60L, 53L,
"Portland", 1L, "Seattle U", 40L, 21L, 19L, 79L, 76L, 106L, 89L, 17L, 84L, 68L, 68L, 64L,
"Saint Martin's", 1L, "Seattle U", 39L, 18L, 21L, 74L, 73L, 132L, 86L, 46L, 98L, 63L, 60L, 69L,
"Utah Valley", 1L, "Seattle U", 44L, 23L, 21L, 92L, 90L, 100L, 103L, -3L, 92L, 93L, 73L, 57L,
"Dixie St.", 0L, "Seattle U", 40L, 21L, 19L, 75L, 73L, 101L, 105L, -4L, 76L, 77L, 71L, 64L,
"Dixie St.", 0L, "Seattle U", 39L, 22L, 18L, 78L, 76L, 99L, 74L, 25L, 77L, 56L, 57L, 57L
)
fdecteamstats = filter(decteamstats, Team == "Seattle U") %>%
mutate(WL = case_when(PTS > dPTS ~ 1, PTS < dPTS ~ 0)) %>%
## place the appended column more to the front
select(ID:Team, WL, everything())
fdecteamstats
#> # A tibble: 11 x 16
#> ID Home Team WL Mins oMins dMins oPOSS dPOSS ORTG DRTG NETRTG
#> <chr> <int> <chr> <dbl> <int> <int> <int> <int> <int> <int> <int> <int>
#> 1 UCLA 0 Seat~ 0 39 20 19 72 72 72 108 -36
#> 2 Long~ 0 Seat~ 0 38 22 17 76 77 99 104 -5
#> 3 Wash~ 0 Seat~ 0 39 21 18 68 69 60 106 -46
#> 4 Nort~ 1 Seat~ 1 39 16 23 70 69 127 58 69
#> 5 Col.~ 1 Seat~ 1 39 19 20 68 70 115 77 38
#> 6 Cali~ 0 Seat~ 0 39 20 19 63 64 103 109 -6
#> 7 Port~ 1 Seat~ 1 40 21 19 79 76 106 89 17
#> 8 Sain~ 1 Seat~ 1 39 18 21 74 73 132 86 46
#> 9 Utah~ 1 Seat~ 0 44 23 21 92 90 100 103 -3
#> 10 Dixi~ 0 Seat~ 0 40 21 19 75 73 101 105 -4
#> 11 Dixi~ 0 Seat~ 1 39 22 18 78 76 99 74 25
#> # ... with 4 more variables: PTS <int>, dPTS <int>, FGA <int>, dFGA <int>
Created on 2021-02-27 by the reprex package (v1.0.0)